1. Field of the Invention
The present invention relates to a method of controlling a vibration motor which generates vibrational waves in an ultrasonic vibrator, and relatively moves a movable body contacting the ultrasonic vibrator by a frictional force.
2. Description of the Related Art
Devices such as cameras or digital camcorders use a vibration motor in which an AC signal is supplied to an electromechanical energy conversion element, and vibrational waves are generated in an ultrasonic vibrator to relatively move a movable body contacting the ultrasonic vibrator by a frictional force. In such a device, an electric power supply source is a battery.
In recent years, the devices such as cameras or digital camcorders are increasingly downsized, and the batteries are also downsized with reduction in a power supply capacity. Various power saving measures are therefore proposed.
For example, Japanese Patent No. 3,140,236 proposes, as illustrated in FIG. 8, an ultrasonic motor driver circuit in which in order to prevent noise from occurring, a pulse width of a drive pulse is changed according to a battery voltage, and if the battery voltage is sufficiently high, the pulse width is narrowed to prevent the supply of unnecessary energy.
Further, a vibration motor driver circuit is known which conducts control so that a motor driving method is changed according to the battery voltage to keep a constant input power.
Further, Japanese Patent Application Laid-Open No. H09-271174 proposes a control device for a vibrational wave actuator, which detects a drive current and controls a pulse width so that the detected value does not exceed an allowable current.
However, the above-mentioned related art suffers from the following problems.
In Japanese Patent No. 3,140,236, the pulse width and the driving method can be changed according to the battery voltage. However, because a detection target is the battery voltage, a current actually consumed by the motor cannot be detected.
For that reason, it is impossible to deal with a case where the current is increased by a change in temperature or a change in drive load. If the current exceeds a given amount that can be supplied by the battery, there arises such a problem that the current more than the given amount cannot be supplied, and the battery voltage decreases with the result that an overall system of the device stops.
Further, in the case where the method as disclosed in Japanese Patent Application Laid-Open No. H09-271174 is used, which detects a current value and controls the pulse width so that the current does not exceed a given value, the following problem arises.
In a “first frequency domain” where a motor output also increases with an increase in input electric power due to a frequency change, a pulse width control and a velocity control are implemented at the same time.
For that reason, the operation of decreasing a frequency for increasing the velocity and the operation of decreasing the pulse width for decreasing the current are mixed, and stable driving cannot be conducted.
Further, there arises such a problem that the frequency exceeds a resonance frequency, thereby causing the motor to stop.
Further, the voltage is increased due to an amplification effect associated with an impedance circuit in a driver circuit which is away from the resonance frequency.
However, in a frequency domain where a current flows into a capacity component side of the vibration motor, that is, a “second frequency domain” where the motor output does not increase with an increase in the input electric power due to the frequency change, the operation is conducted so that the current in an amount that can be always output (that is, to the utmost limit) flows.
For that reason, an ineffective current flowing in the capacity component side always continues to flow, thereby degrading the circuit efficiency.